A researcher at the University of Canterbury has found mortality rates have remained high in areas where there is significant social inequality.
But mortality fell markedly in better-off areas and dropped on average across New Zealand as whole.
The research covered the period from 1980 to 2001.
It was carried out by senior lecturer in geography Jamie Pearce, who says mortality rates in New Zealand as a whole fell 12% in that time.
But the drop was 18% in South Canterbury but only 4% in Northland.
The research attributes the difference to the degree of social inequality in each area.
Dr Pearce also found conditions like heart disease produced less mortality, while diabetes and cancer killed more people.